Duplex transmission of signals within the same frequency band over a link such as a telephone line can be subject to problems due to echo effects, in particular so-called "talker echo" (referred to below simply as echo) where reception by a station of echo versions of its own transmitted signals interferes with its reception function. Echo cancellers are commonly used to alleviate this problem. For the purposes of this description, echo effects will be understood to include any effect caused by a station receiving residues of its own transmission as for example residues due to impedance mismatch, inadequate filtering or, in a radio system, antenna misalignment.
According to the present invention there is provided, a duplex data transmission system comprising first and second stations connected via a transmission path, in which each station has assessment means for assessing received signal quality, at least one station has means for transmitting results of the assessment to the other station, the or each other station has control means operable to estimate a transmitted power adjustment to be made by one station, and at least one station includes means responsive thereto to effect an adjustment to its transmitting power, characterized in that the control means is operable in response to the results of its own assessment and that made by and communicated to it by the other station such that the signal-to-noise ratios of the signals received by the two stations shall be substantially in a predetermined relationship.
Other aspects of the invention are as recited in the claims.